Problem:
The "spill" indicator that ui2 appends when messages overflow the
available height is drawn with whatever highlight the message tail
happens to have, so it is indistinguishable from the message text.
Solution:
Give the [+x] chunks an explicit `MoreMsg` highlight, and only fall back
to the message tail highlight for chunks that don't carry one of their
own.
AI-assisted
06bff27929
